The dynamic range of images captured on one medium (such as negative film) may exceed the dynamic range of the medium that the image is rendered on (such as photographic paper). This results in a loss of image detail in the rendered image. The detail may be lost in the highlight portion of the image (such as when a backlit image is printed light enough to properly render the shadow detail, but rendered too light to show the highlight detail such as the clouds in the sky). Or the detail may be lost in the shadow portion of the image (such as when a “flash-in-the-face” image is rendered properly for the subject of the picture, but the background is rendered too dark to show the detail in the background).
In conventional photographic printing techniques, these problems are addressed by the process of exposure adjustment. The exposure of the image is determined such that the lightness of the image's subject is optimally reproduced on the rendered medium. In conventional digital output processing techniques, the digital image's exposure must also be adjusted for the optimal reproduction of the image's subject in the rendered image. Rendering, in a digital technique, describes the process of mapping the scene colors into the colors of the output medium. The rendered image is the image resulting from a rendering process. Rendering often results in the loss of image information due to many scene colors mapping to a single color in the rendered image.
If the digital image is encoded in a manner such that the code values are related linearly to the scene exposure (i.e., a linearly-encoded digital image), then an exposure adjustment can be digitally applied to the image by scaling the image by a constant. For example, scaling the digital values by a factor Q is similar to scaling the exposure time of the original image capture by a factor Q. If the digital image is encoded in a manner such that the code values are related to the logarithm of the scene exposure (i.e., a log-encoded digital image), then an exposure adjustment can be digitally applied to the image by adding a constant to the image.
The transformation performed by rendering a log-encoded image to the gamut of an output medium, thereby obtaining a rendered image, is a highly nonlinear transformation. The application of simple transforms such as linear functions to the rendered digital image, however, is not equivalent to modifying the exposure on the original scene.
In U.S. Pat. No. 6,285,784, Spaulding et al. describe a method of constructing an extended color gamut digital image from a limited color gamut digital image (such as a rendered image) and a residual image representing a difference between the extended color gamut digital image and the limited color gamut digital image, specifying a desired image modification (such as an exposure adjustment), and then producing a modified limited color gamut digital image. This method is computationally complex and requires a residual image.
In U.S. Pat. No. 5,414,538, Eschbach describes a method of modifying the exposure of an image by applying a gamma function to the image in order to map input signals representing overall input image intensity to overall output signals representing intensity as printed or displayed. This method of modifying image exposure, however, does not visually give an observer the impression that the exposure on the original scene was modified. In U.S. Pat. No. 6,342,951, Eschbach et al. describe a method of gamut mapping utilizing the process of inverting the gray value of each pixel, applying a gamma to the inverted gray values, and inverting each inverted gray value after the application of the gamma value. However, the method employed in by Eschbach et al. is not used to modify the exposure of a rendered image. Therefore there still exists a need to provide an improved method for modifying the exposure of a rendered digital image.